Top-Down Syntheses of Nickel-Based Structured Catalysts for Hydrogen Production from Ammonia

We report the fabrication and catalytic performance evaluation of highly active and stable nickel (Ni)-based structured catalysts for ammonia dehydrogenation with nearly complete conversion using nonprecious metal catalysts. Low-temperature chemical alloying (LTCA) followed by selective aluminum (Al) dealloying was utilized to synthesize foam-type structured catalysts ready for implementation in commercial-scale catalytic reactors. The crystalline phases of Ni-Al alloy (NiAl₃, Ni₂Al₃, or both) in the near-surface layer were controlled by tuning the alloying time. The best-performing catalyst was obtained from a Ni foam substrate with a NiAl₃/Ni₂Al₃ overlayer synthesized by LTCA at 400 °C for 20 h. The developed Ni catalyst exhibited an activity enhancement of 10-fold over the nontreated Ni foam and showed outstanding activities of 15 800 mol_H2mol_Ni^-1h^-1 (TOF: 4.39 s^-1) and 19 978 mol_H2mol_Ni^-1h^-1 (TOF: 5.55 s^-1) at 550 and 600 °C, respectively. This performance is unprecedented compared with previously reported Ni-based ammonia cracking catalysts with higher-end performance (TOFs of 0.08-1.45 s^-1 at 550 °C). Moreover, this catalyst showed excellent stability for 100 h at 600 °C while discharging an extremely low NH₃ concentration of 1034 ppm. The NH₃ concentration in the exhaust gas was further reduced to 690 and 271 ppm at 700 and 800 °C, respectively, while no deactivation was observed at these elevated temperatures. Through material characterizations, we clarified that controlling the degree of Al alloying in the outermost layer of Ni is a crucial factor in determining the activity and stability because residual Al possibly modifies the electronic structure of Ni for enhanced activity as well as transforming to acidic alumina for increased intrinsic activity and stability.